Generally, a thickness of a painted film, plated film or aluminate film which is coated on a metal for purposes of anti-corrosion, insulation or good outside appearance lies within a wide range of values, from a few micrometers to hundreds of micrometers. To measure this thickness without hurting the film, there is an eddy-current instrument which uses a high frequency electromagnetic field affected by what is called an eddy current loss. This eddy current loss is caused in a metal due to the applied high frequency electromagnetic field that is generated from the primary winding wound around a core of a probe which was pressed on the film. It is measured by the secondary winding wound around that probe. The high frequency electromagnetic field is then converted into the film thickness and is displayed. On the other hand, when the metal is an iron based system having a high permeability, the electromagnetic inductive coupling between the probe and the metal becomes strong, so that it is advantageous to constitute a measuring circuit. Such a measuring instrument is known as an electromagnetic instrument for measuring thickness. The invention can be commonly applied to both instruments for measuring thickness of the above-mentioned types.
As the request for quality of the surface processing film on the metal becomes severe, there is demanded an instrument which can more promptly perform the measurement of film thickness at various locations and can obtain a higher arithmetic operation result.
At a construction field such as a bridge, steel tower, high-rise building, etc., when a film thickness is measured using a conventional thickness measuring instrument, in order to see if the thickness lies within a reference range the displayed value regarding the thickness is read out by a measuring person who climbs the measuring location and is recorded by another recording person. This process is repeatedly executed with respect to each measuring point. The data recorded is inputted to a computing apparatus or the like, so that the mean value is obtained, or a discrimination is made to see if it lies within the reference range of thickness. It is very dangerous for the measuring person himself to record the measured data and carry out the arithmetic operation to obtain the mean value at the above-mentioned construction field.